This invention relates to enhanced heat transfer surfaces, such as the surfaces (and particularly the inner surfaces) of heat transfer tubes, that facilitate heat transfer from one side of the surface to the other. Heat transfer tubes are commonly used in equipment, such as, for example, flooded evaporators, falling film evaporators, spray evaporators, absorption chillers, condensers, direct expansion coolers, and single phase coolers and heaters, used in the refrigeration, chemical, petrochemical, and food-processing industries. A variety of heat transfer mediums may be used in these applications, including, but not limited to, pure water, a water glycol mixture, any type of refrigerant (such as R-22, R-134a, R-123, etc.), ammonia, petrochemical fluids, and other mixtures.
An ideal heat transfer tube would allow heat to flow completely uninhibited from the interior of the tube to the exterior of the tube and vice versa. However, such free flow of heat across the tube is generally thwarted by the resistance to heat transfer. The overall resistance of the tube to heat transfer is calculated by adding the individual resistances from the outside to the inside of the tube or vice versa. To improve the heat transfer efficiency of the tube, tube manufacturers have striven to uncover ways to reduce the overall resistance of the tube. One such way is to enhance the outer surface of the tube, such as by forming fins on the outer surface. As a result of recent advances in enhancing the outer tube surface (see, e.g., U.S. Pat. Nos. 5,697,430 and 5,996,686), only a small part of the overall tube resistance is attributable to the outside of the tube. For example, a typical evaporator tube used in a flooded chiller with an enhanced outer surface but smooth inner surface typically has a 10:1 inner resistance:outer resistance ratio. Ideally, one wants to obtain an inside to outside resistance ratio of 1:1. It becomes all the more important, therefore, to develop enhancements to the inner surface of the tube that will significantly reduce the tube side resistance and improve overall heat transfer performance of the tube.
It is known to provide heat transfer tubes with alternating grooves and ridges on their inner surfaces. The grooves and ridges cooperate to enhance turbulence of fluid heat transfer mediums, such as water, delivered within the tube. This turbulence increases the fluid mixing close to the inner tube surface to reduce or virtually eliminate the boundary layer build-up of the fluid medium close to the inner surface of the tube. The boundary layer thermal resistance significantly detracts from heat transfer performance by increasing the heat transfer resistance of the tube. The grooves and ridges also provide extra surface area for additional heat exchange. This basic premise is taught in U.S. Pat. No. 3,847,212 to Withers, Jr. et al.
The pattern, shapes and sizes of the grooves and ridges on the inner tube surface may be changed to further increase heat exchange performance. To that end, tube manufacturers have gone to great expense to experiment with alternative designs, including those disclosed in U.S. Pat. No. 5,791,405 to Takima et al., U.S. Pat. Nos. 5,332,034 and 5,458,191 to Chiang et al., and U.S. Pat. No. 5,975,196 to Gaffaney et al.
Moreover, some types of heat transfer surfaces work by using the phase change of a liquid to absorb heat. Thus, heat transfer surfaces often incorporate a surface for enhancing boiling or evaporating. It is generally known that the heat transfer performance of a surface can be enhanced by increasing nucleation sites on the boiling surfaces, by inducing agitation near a single-phase heat transfer surface, or by increasing area and surface tension effects on condensation surfaces. One method for enhancing boiling or evaporating is to roughen the heat transfer surface by sintering, radiation-melting or edging methods to form a porous layer thereon. A heat transfer surface having such a porous layer is known to exhibit better heat transfer characteristics than that of a smooth surface. However, the voids or cells formed by the above-mentioned methods are small and impurities contained in the boiling liquid may clog them so that the heat transfer performance of the surface is impaired. Additionally, since the voids or cells formed are non-uniform in size or dimension, the heat transfer performance may vary along the surface. Furthermore, known heat transfer tubes incorporating boiling or evaporating surfaces often require multiple steps or passes with tools to create the final surface.
Tube manufacturers have gone to great expense to experiment with alternative designs including those disclosed in U.S. Pat. No. 4,561,497 to Nakajima et al., U.S. Pat. No. 4,602,681 to Daikoku et al., U.S. Pat. No. 4,606,405 to Nakayama et al., U.S. Pat. No. 4,653,163 to Kuwahara et al., U.S. Pat. No. 4,678,029 to Sasaki et al., U.S. Pat. No. 4,794,984 to Lin and U.S. Pat. No. 5,351,397 to Angeli.
While all of these surface designs aim to improve the heat transfer performance of the surface, there remains a need in the industry to continue to improve upon tube designs by modifying existing and creating new designs that enhance heat transfer performance. Additionally, a need also exists to create designs and patterns that can be transferred onto heat transfer surfaces more quickly and cost-effectively. As described below, the geometries of the heat transfer surfaces of the invention, as well as tools to form those geometries, have significantly improved heat transfer performance.